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This should be really easy but I can't seem to find the way to do it, thanks to the great documentation. I'm thinking I'm missing something big.

I have a function that buys ownership of something. I want to remove amount ether to msg.sender (user a) and give them to owner (user b) (minus fees).

Can I do it directly? Do I have to first let a send ether to the contract and then send b the same amount, or is there a function to transfer directly?

I assume owner.transfer(amount) (or .send(amount)) sends ether from the contract to the owner, but then how do I get ether from a?

I'm using Solidity by Example as a model, but in the Blind Auction example I can't seem to find what function actually sends ether from the bidder to the contract.

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You can't just take somebody's ether, so yes, they need to send it to the contract first, but this can all be done in a single contract call. E.g.

// "payable" means ether can be sent to this function
function buyIt() public payable {
    // msg.value is how much ether was sent
    require(msg.value == price);

    // send the ether to "owner"
    owner.transfer(msg.value);

    // msg.sender is the new "owner"
    owner = msg.sender;
}

When calling this function, the caller must attach price ether. This means setting the "value" field of the transaction.

I think this blog post will help: https://programtheblockchain.com/posts/2017/12/15/writing-a-contract-that-handles-ether/.

EDIT

You linked to a very old version of the Solidity documentation, before payable was required to allow ether to be sent. The current version shows a payable bid function. That's where ether is transferred to the contract.

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  • Thanks, I didn't know that you could change the version on readthedocs! With this function, the price is chosen by the buyer, right? If the price is fixed, do I have to check that he sent more than price and then refund all the exceeding ether, or can something handle that automatically? – Teleporting Goat Mar 1 '18 at 9:53
  • I just made up price. I assumed a fixed price. The code I shared requires that the caller send exactly the right amount. If you want to allow overpayment and refund the excess, you could do require(msg.value >= price); if (msg.value > price) { msg.sender.transfer(msg.value - price); }. – user19510 Mar 1 '18 at 9:57
  • Oh my bad, I missed the part with require. It's crystal clear now! :) – Teleporting Goat Mar 1 '18 at 10:01
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How about web3 sendTransaction method? it has "to" option.

https://github.com/ethereum/wiki/wiki/JavaScript-API#web3ethsendtransaction

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    It needs to be inside the contract, I can't do this just at JavaScript level. – Teleporting Goat Jul 12 '18 at 9:19
  • I got it! exactly! We need to go through contract – Kosuke Yoshimura Jul 12 '18 at 9:22

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